Battery capacity is rated in terms of power, specified in units of amp-hours (A-h), as a measure for a battery to deliver x Amps for y hours. To measure the duration of time a device will operate with a specified battery requires measuring the instantaneous currents (Amps) drawn by the device accumulated over time (hours) until the battery is fully discharged.
Commercial integrated circuits exist to monitor battery capacity in cellular phones and other handheld devices. However, these circuits only perform well with devices that draw a fairly constant, slow-changing current from the battery. Such circuits typically fail to give accurate information for devices that rapidly change current consumption over a large range of current. Furthermore, conventional monitoring circuits typically do not maintain accurate calibrations.
Processors that change current consumption over time and across several orders of magnitude now control many devices. In such devices, conventional battery monitoring circuits become inadequate. For example, a conventional battery-monitoring circuit may be specified as having the following operating parameters:                Gvf=32.55 Hz/volt (voltage to frequency gain);        Vsense=100 millivolts (maximum sense voltage);        f=Gvf×Vsense=3.255 Hz (maximum v-f frequency);where Vsense=is the voltage across a known value of resistance conducting an input current. Currents that change more rapidly than 3.255 Hz are not accurately tracked. Since many integrated circuits can turn on in sub-millisecond time intervals, a very fast voltage-to-frequency converter is required to operate at, say, 100,000 times faster than such conventional battery-monitoring circuits.        
Also, such conventional circuits are typically not capable of accurately tracking large changes in the current being monitored. For a 1-ohm sensing resistor conducting about 100 milliamps, the allowable sense voltage attains the maximum value of 100 millivolts and a representative output frequency of 3.255 Hz. However, if the input current decreases to 100 microamps, then the output frequency reduces to 0.003255 Hz which has a periodicity of over 300 seconds that is difficult to measure without complex circuitry.